USE的循环依赖是怎样产生的？

Gentoo / # emerge xorg-server -av
 * Overlay eclass overrides eclass from PORTDIR:
 * 
 * '/usr/local/overlays/xfce4.git/eclass/xfce4.eclass'
 * 
 * It is best to avoid overriding eclasses from PORTDIR because it will
 * trigger invalidation of cached ebuild metadata that is distributed with
 * the portage tree. If you must override eclasses from PORTDIR then you
 * are advised to add FEATURES="metadata-transfer" to /etc/make.conf and to
 * run `emerge --regen` after each time that you run `emerge --sync`. Set
 * PORTAGE_ECLASS_WARNING_ENABLE="0" in /etc/make.conf if you would like to
 * disable this warning.

These are the packages that would be merged, in order:

Calculating dependencies... done!


[nomerge ] x11-base/xorg-server-1.5.3-r1 USE="hal nptl sdl xorg -3dfx -debug -dmx -ipv6 -kdrive -minimal -tslib" INPUT_DEVICES="keyboard mouse -acecad -aiptek -calcomp -citron -digitaledge -dmc -dynapro -elo2300 -elographics -evdev -fpit -hyperpen -jamstudio -joystick -magellan -microtouch -mutouch -palmax -penmount -spaceorb -summa -synaptics -tek4957 -tslib -ur98 -vmmouse -void -wacom" VIDEO_CARDS="nv nvidia -apm -ark -ast -chips -cirrus -cyrix -dummy -epson -fbdev -fglrx -geode -glint -i128 -i740 (-impact) -imstt -intel -mach64 -mga -neomagic (-newport) -nsc -r128 -radeon -radeonhd -rendition -s3 -s3virge -savage -siliconmotion -sis -sisusb (-sunbw2) (-suncg14) (-suncg3) (-suncg6) (-sunffb) (-sunleo) (-suntcx) -tdfx -tga -trident -tseng -v4l -vermilion -vesa -vga -via -vmware -voodoo -xgi" 
[ebuild N ] sys-apps/hal-0.5.11-r6 USE="X acpi crypt -apm -debug -dell -disk-partition -doc -laptop (-selinux)" 
[ebuild N ] dev-python/pyxf86config-0.3.34-r1 
[ebuild N ] x11-base/xorg-server-1.5.3-r1 USE="hal nptl sdl xorg -3dfx -debug -dmx -ipv6 -kdrive -minimal -tslib" INPUT_DEVICES="keyboard mouse -acecad -aiptek -calcomp -citron -digitaledge -dmc -dynapro -elo2300 -elographics -evdev -fpit -hyperpen -jamstudio -joystick -magellan -microtouch -mutouch -palmax -penmount -spaceorb -summa -synaptics -tek4957 -tslib -ur98 -vmmouse -void -wacom" VIDEO_CARDS="nv nvidia -apm -ark -ast -chips -cirrus -cyrix -dummy -epson -fbdev -fglrx -geode -glint -i128 -i740 (-impact) -imstt -intel -mach64 -mga -neomagic (-newport) -nsc -r128 -radeon -radeonhd -rendition -s3 -s3virge -savage -siliconmotion -sis -sisusb (-sunbw2) (-suncg14) (-suncg3) (-suncg6) (-sunffb) (-sunleo) (-suntcx) -tdfx -tga -trident -tseng -v4l -vermilion -vesa -vga -via -vmware -voodoo -xgi" 

 * Error: circular dependencies:

('ebuild', '/', 'x11-base/xorg-server-1.5.3-r1', 'merge') depends on
 ('ebuild', '/', 'sys-apps/hal-0.5.11-r6', 'merge') (hard)
('ebuild', '/', 'sys-apps/hal-0.5.11-r6', 'merge') depends on
 ('ebuild', '/', 'dev-python/pyxf86config-0.3.34-r1', 'merge') (hard)
('ebuild', '/', 'dev-python/pyxf86config-0.3.34-r1', 'merge') depends on
 ('ebuild', '/', 'x11-base/xorg-server-1.5.3-r1', 'merge') (hard)

 * Note that circular dependencies can often be avoided by temporarily
 * disabling USE flags that trigger optional dependencies.